Sum frequency generation spectroscopy and microscopy

Research Summary

The properties of catalysts and electronic devices are determined by the structure and properties of their surfaces and buried interfaces. In order to understand their dynamic behavior, it is essential to investigate the geometric and electronic structures of these surfaces and interfaces. Conventional surface analysis methods require homogeneous and defect-free surfaces or interfaces for use as models for more complicated real systems. However, spatially inhomogeneous structures at the surfaces or interfaces often have a large influence on the properties of the catalysts or devices. Thus, spatially resolved observation of the molecular structures is important. For that purpose, we have developed a sum frequency generation microscopy system.

4-1) Sum frequency generation microscopy

When infrared and visible laser beams are focused on a sample simultaneously, a new beam is generated whose frequency is the sum of those of the two incident beams. This process is called infrared-visible sum frequency generation (SFG). SFG occurs only at interfaces of different materials or at surfaces because of the symmetry selection rules. By varying the wavelength of the infrared beam, vibrational spectra of the molecules at the interfaces or surfaces can be selectively obtained without any influence from the bulk materials.

Conventional SFG techniques provide only spatially averaged information. With our SFG microscopy system, spatially resolved SFG spectra can be obtained, where the SFG beam generated from the sample is expanded by several lenses and imaged using a CCD camera (Fig. 4-1-1). The spatial resolution of this technique is 5 µm. Figure 4-1-2 shows an SFG image of a molecular monolayer patterned on a gold surface. A clear striped pattern is observed through vibrational resonance. Time-resolved measurements are also possible because picosecond pulsed lasers are used in the system. Thus, the time evolution of spatial patterns in the vibrational spectra can be obtained.

Fig. 4-1-1 Detection method in SFG microscopy.

Fig. 4-1-2 SFG image of patterned self-assembled monolayer on gold surface.

4-2) Observation of buried interfaces in organic electronic devices

Electronic devices fabricated with organic molecules are attracting growing attention because of their structural flexibility and simplicity of processing. The organic field effect transistor (OFET) is an important element of organic electronics. A schematic drawing of an OFET is shown in Fig. 4-2-1(a). When a certain voltage is applied to the gate electrode, an electric current is generated between the source and drain electrodes. Such transistors are often used as switching devices. Although the mobility of OFETs has been improved significantly and is now comparable to that of silicon-based devices, they still suffer from serious drawbacks such as short lifetimes and irreproducible performance. In order to overcome these drawbacks, it is important to understand the mechanisms of carrier generation and transport. Previous studies have revealed that carriers are injected into the interface between the gate electrode and the organic semiconductor layer upon application of the gate voltage, and these carriers are responsible for the source-drain current. Investigation of the spatial distribution of the carrier density and the molecular structure during the operation of OFETs will provide detailed information on the mechanisms of carrier injection and transport. For this reason, we are applying SFG microscopy to observe the buried interfaces of OFETs, since its interface selectivity offers advantages for this type of study.

Fig. 4-2-1 (a) Schematic drawing of OFET and (b) the structures of typical organic semiconductor molecules consisting OFETs.

References

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